
<!DOCTYPE html>

<html xmlns="http://www.w3.org/1999/xhtml">
  <head>
    <meta charset="utf-8" />
    <title>abc — Abstract Base Classes &#8212; Python 3.8.13 documentation</title><meta name="viewport" content="width=device-width, initial-scale=1.0">
    
    <link rel="stylesheet" href="../_static/pydoctheme.css?2022.1" type="text/css" />
    <link rel="stylesheet" href="../_static/pygments.css" type="text/css" />
    
    <script id="documentation_options" data-url_root="../" src="../_static/documentation_options.js"></script>
    <script src="../_static/jquery.js"></script>
    <script src="../_static/underscore.js"></script>
    <script src="../_static/doctools.js"></script>
    <script src="../_static/language_data.js"></script>
    
    <script src="../_static/sidebar.js"></script>
    
    <link rel="search" type="application/opensearchdescription+xml"
          title="Search within Python 3.8.13 documentation"
          href="../_static/opensearch.xml"/>
    <link rel="author" title="About these documents" href="../about.html" />
    <link rel="index" title="Index" href="../genindex.html" />
    <link rel="search" title="Search" href="../search.html" />
    <link rel="copyright" title="Copyright" href="../copyright.html" />
    <link rel="next" title="atexit — Exit handlers" href="atexit.html" />
    <link rel="prev" title="contextlib — Utilities for with-statement contexts" href="contextlib.html" />
    <link rel="canonical" href="https://docs.python.org/3/library/abc.html" />
    
      
    

    
    <style>
      @media only screen {
        table.full-width-table {
            width: 100%;
        }
      }
    </style>
<link rel="shortcut icon" type="image/png" href="../_static/py.svg" />
            <script type="text/javascript" src="../_static/copybutton.js"></script>
            <script type="text/javascript" src="../_static/menu.js"></script> 

  </head>
<body>
<div class="mobile-nav">
    <input type="checkbox" id="menuToggler" class="toggler__input" aria-controls="navigation"
           aria-pressed="false" aria-expanded="false" role="button" aria-label="Menu" />
    <label for="menuToggler" class="toggler__label">
        <span></span>
    </label>
    <nav class="nav-content" role="navigation">
         <a href="https://www.python.org/" class="nav-logo">
             <img src="../_static/py.svg" alt="Logo"/>
         </a>
        <div class="version_switcher_placeholder"></div>
        <form role="search" class="search" action="../search.html" method="get">
            <svg xmlns="http://www.w3.org/2000/svg" width="20" height="20" viewBox="0 0 24 24" class="search-icon">
                <path fill-rule="nonzero"
                        d="M15.5 14h-.79l-.28-.27a6.5 6.5 0 001.48-5.34c-.47-2.78-2.79-5-5.59-5.34a6.505 6.505 0 00-7.27 7.27c.34 2.8 2.56 5.12 5.34 5.59a6.5 6.5 0 005.34-1.48l.27.28v.79l4.25 4.25c.41.41 1.08.41 1.49 0 .41-.41.41-1.08 0-1.49L15.5 14zm-6 0C7.01 14 5 11.99 5 9.5S7.01 5 9.5 5 14 7.01 14 9.5 11.99 14 9.5 14z" fill="#444"></path>
            </svg>
            <input type="text" name="q" aria-label="Quick search"/>
            <input type="submit" value="Go"/>
        </form>
    </nav>
    <div class="menu-wrapper">
        <nav class="menu" role="navigation" aria-label="main navigation">
            <div class="language_switcher_placeholder"></div>
  <h4>Previous topic</h4>
  <p class="topless"><a href="contextlib.html"
                        title="previous chapter"><code class="xref py py-mod docutils literal notranslate"><span class="pre">contextlib</span></code> — Utilities for <code class="xref std std-keyword docutils literal notranslate"><span class="pre">with</span></code>-statement contexts</a></p>
  <h4>Next topic</h4>
  <p class="topless"><a href="atexit.html"
                        title="next chapter"><code class="xref py py-mod docutils literal notranslate"><span class="pre">atexit</span></code> — Exit handlers</a></p>
  <div role="note" aria-label="source link">
    <h3>This Page</h3>
    <ul class="this-page-menu">
      <li><a href="../bugs.html">Report a Bug</a></li>
      <li>
        <a href="https://github.com/python/cpython/blob/3.8/Doc/library/abc.rst"
            rel="nofollow">Show Source
        </a>
      </li>
    </ul>
  </div>
        </nav>
    </div>
</div>

  
    <div class="related" role="navigation" aria-label="related navigation">
      <h3>Navigation</h3>
      <ul>
        <li class="right" style="margin-right: 10px">
          <a href="../genindex.html" title="General Index"
             accesskey="I">index</a></li>
        <li class="right" >
          <a href="../py-modindex.html" title="Python Module Index"
             >modules</a> |</li>
        <li class="right" >
          <a href="atexit.html" title="atexit — Exit handlers"
             accesskey="N">next</a> |</li>
        <li class="right" >
          <a href="contextlib.html" title="contextlib — Utilities for with-statement contexts"
             accesskey="P">previous</a> |</li>

          <li><img src="../_static/py.svg" alt="python logo" style="vertical-align: middle; margin-top: -1px"/></li>
          <li><a href="https://www.python.org/">Python</a> &#187;</li>
          <li class="switchers">
            <div class="language_switcher_placeholder"></div>
            <div class="version_switcher_placeholder"></div>
          </li>
          <li>
              
          </li>
    <li id="cpython-language-and-version">
      <a href="../index.html">3.8.13 Documentation</a> &#187;
    </li>

          <li class="nav-item nav-item-1"><a href="index.html" >The Python Standard Library</a> &#187;</li>
          <li class="nav-item nav-item-2"><a href="python.html" accesskey="U">Python Runtime Services</a> &#187;</li>
                <li class="right">
                    

    <div class="inline-search" role="search">
        <form class="inline-search" action="../search.html" method="get">
          <input placeholder="Quick search" aria-label="Quick search" type="text" name="q" />
          <input type="submit" value="Go" />
          <input type="hidden" name="check_keywords" value="yes" />
          <input type="hidden" name="area" value="default" />
        </form>
    </div>
                     |
                </li>
            
      </ul>
    </div>    

    <div class="document">
      <div class="documentwrapper">
        <div class="bodywrapper">
          <div class="body" role="main">
            
  <div class="section" id="module-abc">
<span id="abc-abstract-base-classes"></span><h1><a class="reference internal" href="#module-abc" title="abc: Abstract base classes according to :pep:`3119`."><code class="xref py py-mod docutils literal notranslate"><span class="pre">abc</span></code></a> — Abstract Base Classes<a class="headerlink" href="#module-abc" title="Permalink to this headline">¶</a></h1>
<p><strong>Source code:</strong> <a class="reference external" href="https://github.com/python/cpython/tree/3.8/Lib/abc.py">Lib/abc.py</a></p>
<hr class="docutils" />
<p>This module provides the infrastructure for defining <a class="reference internal" href="../glossary.html#term-abstract-base-class"><span class="xref std std-term">abstract base
classes</span></a> (ABCs) in Python, as outlined in <span class="target" id="index-0"></span><a class="pep reference external" href="https://www.python.org/dev/peps/pep-3119"><strong>PEP 3119</strong></a>;
see the PEP for why this was added to Python. (See also <span class="target" id="index-1"></span><a class="pep reference external" href="https://www.python.org/dev/peps/pep-3141"><strong>PEP 3141</strong></a> and the
<a class="reference internal" href="numbers.html#module-numbers" title="numbers: Numeric abstract base classes (Complex, Real, Integral, etc.)."><code class="xref py py-mod docutils literal notranslate"><span class="pre">numbers</span></code></a> module regarding a type hierarchy for numbers based on ABCs.)</p>
<p>The <a class="reference internal" href="collections.html#module-collections" title="collections: Container datatypes"><code class="xref py py-mod docutils literal notranslate"><span class="pre">collections</span></code></a> module has some concrete classes that derive from
ABCs; these can, of course, be further derived. In addition, the
<a class="reference internal" href="collections.abc.html#module-collections.abc" title="collections.abc: Abstract base classes for containers"><code class="xref py py-mod docutils literal notranslate"><span class="pre">collections.abc</span></code></a> submodule has some ABCs that can be used to test whether
a class or instance provides a particular interface, for example, if it is
hashable or if it is a mapping.</p>
<p>This module provides the metaclass <a class="reference internal" href="#abc.ABCMeta" title="abc.ABCMeta"><code class="xref py py-class docutils literal notranslate"><span class="pre">ABCMeta</span></code></a> for defining ABCs and
a helper class <a class="reference internal" href="#abc.ABC" title="abc.ABC"><code class="xref py py-class docutils literal notranslate"><span class="pre">ABC</span></code></a> to alternatively define ABCs through inheritance:</p>
<dl class="class">
<dt id="abc.ABC">
<em class="property">class </em><code class="sig-prename descclassname">abc.</code><code class="sig-name descname">ABC</code><a class="headerlink" href="#abc.ABC" title="Permalink to this definition">¶</a></dt>
<dd><p>A helper class that has <a class="reference internal" href="#abc.ABCMeta" title="abc.ABCMeta"><code class="xref py py-class docutils literal notranslate"><span class="pre">ABCMeta</span></code></a> as its metaclass.  With this class,
an abstract base class can be created by simply deriving from <a class="reference internal" href="#abc.ABC" title="abc.ABC"><code class="xref py py-class docutils literal notranslate"><span class="pre">ABC</span></code></a>
avoiding sometimes confusing metaclass usage, for example:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="kn">from</span> <span class="nn">abc</span> <span class="kn">import</span> <span class="n">ABC</span>

<span class="k">class</span> <span class="nc">MyABC</span><span class="p">(</span><span class="n">ABC</span><span class="p">):</span>
    <span class="k">pass</span>
</pre></div>
</div>
<p>Note that the type of <a class="reference internal" href="#abc.ABC" title="abc.ABC"><code class="xref py py-class docutils literal notranslate"><span class="pre">ABC</span></code></a> is still <a class="reference internal" href="#abc.ABCMeta" title="abc.ABCMeta"><code class="xref py py-class docutils literal notranslate"><span class="pre">ABCMeta</span></code></a>, therefore
inheriting from <a class="reference internal" href="#abc.ABC" title="abc.ABC"><code class="xref py py-class docutils literal notranslate"><span class="pre">ABC</span></code></a> requires the usual precautions regarding
metaclass usage, as multiple inheritance may lead to metaclass conflicts.
One may also define an abstract base class by passing the metaclass
keyword and using <a class="reference internal" href="#abc.ABCMeta" title="abc.ABCMeta"><code class="xref py py-class docutils literal notranslate"><span class="pre">ABCMeta</span></code></a> directly, for example:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="kn">from</span> <span class="nn">abc</span> <span class="kn">import</span> <span class="n">ABCMeta</span>

<span class="k">class</span> <span class="nc">MyABC</span><span class="p">(</span><span class="n">metaclass</span><span class="o">=</span><span class="n">ABCMeta</span><span class="p">):</span>
    <span class="k">pass</span>
</pre></div>
</div>
<div class="versionadded">
<p><span class="versionmodified added">New in version 3.4.</span></p>
</div>
</dd></dl>

<dl class="class">
<dt id="abc.ABCMeta">
<em class="property">class </em><code class="sig-prename descclassname">abc.</code><code class="sig-name descname">ABCMeta</code><a class="headerlink" href="#abc.ABCMeta" title="Permalink to this definition">¶</a></dt>
<dd><p>Metaclass for defining Abstract Base Classes (ABCs).</p>
<p>Use this metaclass to create an ABC.  An ABC can be subclassed directly, and
then acts as a mix-in class.  You can also register unrelated concrete
classes (even built-in classes) and unrelated ABCs as “virtual subclasses” –
these and their descendants will be considered subclasses of the registering
ABC by the built-in <a class="reference internal" href="functions.html#issubclass" title="issubclass"><code class="xref py py-func docutils literal notranslate"><span class="pre">issubclass()</span></code></a> function, but the registering ABC
won’t show up in their MRO (Method Resolution Order) nor will method
implementations defined by the registering ABC be callable (not even via
<a class="reference internal" href="functions.html#super" title="super"><code class="xref py py-func docutils literal notranslate"><span class="pre">super()</span></code></a>). <a class="footnote-reference brackets" href="#id2" id="id1">1</a></p>
<p>Classes created with a metaclass of <a class="reference internal" href="#abc.ABCMeta" title="abc.ABCMeta"><code class="xref py py-class docutils literal notranslate"><span class="pre">ABCMeta</span></code></a> have the following method:</p>
<dl class="method">
<dt id="abc.ABCMeta.register">
<code class="sig-name descname">register</code><span class="sig-paren">(</span><em class="sig-param">subclass</em><span class="sig-paren">)</span><a class="headerlink" href="#abc.ABCMeta.register" title="Permalink to this definition">¶</a></dt>
<dd><p>Register <em>subclass</em> as a “virtual subclass” of this ABC. For
example:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="kn">from</span> <span class="nn">abc</span> <span class="kn">import</span> <span class="n">ABC</span>

<span class="k">class</span> <span class="nc">MyABC</span><span class="p">(</span><span class="n">ABC</span><span class="p">):</span>
    <span class="k">pass</span>

<span class="n">MyABC</span><span class="o">.</span><span class="n">register</span><span class="p">(</span><span class="nb">tuple</span><span class="p">)</span>

<span class="k">assert</span> <span class="nb">issubclass</span><span class="p">(</span><span class="nb">tuple</span><span class="p">,</span> <span class="n">MyABC</span><span class="p">)</span>
<span class="k">assert</span> <span class="nb">isinstance</span><span class="p">((),</span> <span class="n">MyABC</span><span class="p">)</span>
</pre></div>
</div>
<div class="versionchanged">
<p><span class="versionmodified changed">Changed in version 3.3: </span>Returns the registered subclass, to allow usage as a class decorator.</p>
</div>
<div class="versionchanged">
<p><span class="versionmodified changed">Changed in version 3.4: </span>To detect calls to <a class="reference internal" href="#abc.ABCMeta.register" title="abc.ABCMeta.register"><code class="xref py py-meth docutils literal notranslate"><span class="pre">register()</span></code></a>, you can use the
<a class="reference internal" href="#abc.get_cache_token" title="abc.get_cache_token"><code class="xref py py-func docutils literal notranslate"><span class="pre">get_cache_token()</span></code></a> function.</p>
</div>
</dd></dl>

<p>You can also override this method in an abstract base class:</p>
<dl class="method">
<dt id="abc.ABCMeta.__subclasshook__">
<code class="sig-name descname">__subclasshook__</code><span class="sig-paren">(</span><em class="sig-param">subclass</em><span class="sig-paren">)</span><a class="headerlink" href="#abc.ABCMeta.__subclasshook__" title="Permalink to this definition">¶</a></dt>
<dd><p>(Must be defined as a class method.)</p>
<p>Check whether <em>subclass</em> is considered a subclass of this ABC.  This means
that you can customize the behavior of <code class="docutils literal notranslate"><span class="pre">issubclass</span></code> further without the
need to call <a class="reference internal" href="#abc.ABCMeta.register" title="abc.ABCMeta.register"><code class="xref py py-meth docutils literal notranslate"><span class="pre">register()</span></code></a> on every class you want to consider a
subclass of the ABC.  (This class method is called from the
<code class="xref py py-meth docutils literal notranslate"><span class="pre">__subclasscheck__()</span></code> method of the ABC.)</p>
<p>This method should return <code class="docutils literal notranslate"><span class="pre">True</span></code>, <code class="docutils literal notranslate"><span class="pre">False</span></code> or <code class="docutils literal notranslate"><span class="pre">NotImplemented</span></code>.  If
it returns <code class="docutils literal notranslate"><span class="pre">True</span></code>, the <em>subclass</em> is considered a subclass of this ABC.
If it returns <code class="docutils literal notranslate"><span class="pre">False</span></code>, the <em>subclass</em> is not considered a subclass of
this ABC, even if it would normally be one.  If it returns
<code class="docutils literal notranslate"><span class="pre">NotImplemented</span></code>, the subclass check is continued with the usual
mechanism.</p>
</dd></dl>

<p>For a demonstration of these concepts, look at this example ABC definition:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="k">class</span> <span class="nc">Foo</span><span class="p">:</span>
    <span class="k">def</span> <span class="fm">__getitem__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">index</span><span class="p">):</span>
        <span class="o">...</span>
    <span class="k">def</span> <span class="fm">__len__</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
        <span class="o">...</span>
    <span class="k">def</span> <span class="nf">get_iterator</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
        <span class="k">return</span> <span class="nb">iter</span><span class="p">(</span><span class="bp">self</span><span class="p">)</span>

<span class="k">class</span> <span class="nc">MyIterable</span><span class="p">(</span><span class="n">ABC</span><span class="p">):</span>

    <span class="nd">@abstractmethod</span>
    <span class="k">def</span> <span class="fm">__iter__</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
        <span class="k">while</span> <span class="kc">False</span><span class="p">:</span>
            <span class="k">yield</span> <span class="kc">None</span>

    <span class="k">def</span> <span class="nf">get_iterator</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
        <span class="k">return</span> <span class="bp">self</span><span class="o">.</span><span class="fm">__iter__</span><span class="p">()</span>

    <span class="nd">@classmethod</span>
    <span class="k">def</span> <span class="nf">__subclasshook__</span><span class="p">(</span><span class="bp">cls</span><span class="p">,</span> <span class="n">C</span><span class="p">):</span>
        <span class="k">if</span> <span class="bp">cls</span> <span class="ow">is</span> <span class="n">MyIterable</span><span class="p">:</span>
            <span class="k">if</span> <span class="nb">any</span><span class="p">(</span><span class="s2">&quot;__iter__&quot;</span> <span class="ow">in</span> <span class="n">B</span><span class="o">.</span><span class="vm">__dict__</span> <span class="k">for</span> <span class="n">B</span> <span class="ow">in</span> <span class="n">C</span><span class="o">.</span><span class="vm">__mro__</span><span class="p">):</span>
                <span class="k">return</span> <span class="kc">True</span>
        <span class="k">return</span> <span class="bp">NotImplemented</span>

<span class="n">MyIterable</span><span class="o">.</span><span class="n">register</span><span class="p">(</span><span class="n">Foo</span><span class="p">)</span>
</pre></div>
</div>
<p>The ABC <code class="docutils literal notranslate"><span class="pre">MyIterable</span></code> defines the standard iterable method,
<a class="reference internal" href="stdtypes.html#iterator.__iter__" title="iterator.__iter__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__iter__()</span></code></a>, as an abstract method.  The implementation given
here can still be called from subclasses.  The <code class="xref py py-meth docutils literal notranslate"><span class="pre">get_iterator()</span></code> method
is also part of the <code class="docutils literal notranslate"><span class="pre">MyIterable</span></code> abstract base class, but it does not have
to be overridden in non-abstract derived classes.</p>
<p>The <a class="reference internal" href="#abc.ABCMeta.__subclasshook__" title="abc.ABCMeta.__subclasshook__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__subclasshook__()</span></code></a> class method defined here says that any class
that has an <a class="reference internal" href="stdtypes.html#iterator.__iter__" title="iterator.__iter__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__iter__()</span></code></a> method in its
<a class="reference internal" href="stdtypes.html#object.__dict__" title="object.__dict__"><code class="xref py py-attr docutils literal notranslate"><span class="pre">__dict__</span></code></a> (or in that of one of its base classes, accessed
via the <a class="reference internal" href="stdtypes.html#class.__mro__" title="class.__mro__"><code class="xref py py-attr docutils literal notranslate"><span class="pre">__mro__</span></code></a> list) is considered a <code class="docutils literal notranslate"><span class="pre">MyIterable</span></code> too.</p>
<p>Finally, the last line makes <code class="docutils literal notranslate"><span class="pre">Foo</span></code> a virtual subclass of <code class="docutils literal notranslate"><span class="pre">MyIterable</span></code>,
even though it does not define an <a class="reference internal" href="stdtypes.html#iterator.__iter__" title="iterator.__iter__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__iter__()</span></code></a> method (it uses
the old-style iterable protocol, defined in terms of <a class="reference internal" href="../reference/datamodel.html#object.__len__" title="object.__len__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__len__()</span></code></a> and
<a class="reference internal" href="../reference/datamodel.html#object.__getitem__" title="object.__getitem__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__getitem__()</span></code></a>).  Note that this will not make <code class="docutils literal notranslate"><span class="pre">get_iterator</span></code>
available as a method of <code class="docutils literal notranslate"><span class="pre">Foo</span></code>, so it is provided separately.</p>
</dd></dl>

<p>The <a class="reference internal" href="#module-abc" title="abc: Abstract base classes according to :pep:`3119`."><code class="xref py py-mod docutils literal notranslate"><span class="pre">abc</span></code></a> module also provides the following decorator:</p>
<dl class="function">
<dt id="abc.abstractmethod">
<code class="sig-prename descclassname">&#64;</code><code class="sig-prename descclassname">abc.</code><code class="sig-name descname">abstractmethod</code><a class="headerlink" href="#abc.abstractmethod" title="Permalink to this definition">¶</a></dt>
<dd><p>A decorator indicating abstract methods.</p>
<p>Using this decorator requires that the class’s metaclass is <a class="reference internal" href="#abc.ABCMeta" title="abc.ABCMeta"><code class="xref py py-class docutils literal notranslate"><span class="pre">ABCMeta</span></code></a>
or is derived from it.  A class that has a metaclass derived from
<a class="reference internal" href="#abc.ABCMeta" title="abc.ABCMeta"><code class="xref py py-class docutils literal notranslate"><span class="pre">ABCMeta</span></code></a> cannot be instantiated unless all of its abstract methods
and properties are overridden.  The abstract methods can be called using any
of the normal ‘super’ call mechanisms.  <a class="reference internal" href="#abc.abstractmethod" title="abc.abstractmethod"><code class="xref py py-func docutils literal notranslate"><span class="pre">abstractmethod()</span></code></a> may be used
to declare abstract methods for properties and descriptors.</p>
<p>Dynamically adding abstract methods to a class, or attempting to modify the
abstraction status of a method or class once it is created, are not
supported.  The <a class="reference internal" href="#abc.abstractmethod" title="abc.abstractmethod"><code class="xref py py-func docutils literal notranslate"><span class="pre">abstractmethod()</span></code></a> only affects subclasses derived using
regular inheritance; “virtual subclasses” registered with the ABC’s
<code class="xref py py-meth docutils literal notranslate"><span class="pre">register()</span></code> method are not affected.</p>
<p>When <a class="reference internal" href="#abc.abstractmethod" title="abc.abstractmethod"><code class="xref py py-func docutils literal notranslate"><span class="pre">abstractmethod()</span></code></a> is applied in combination with other method
descriptors, it should be applied as the innermost decorator, as shown in
the following usage examples:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="k">class</span> <span class="nc">C</span><span class="p">(</span><span class="n">ABC</span><span class="p">):</span>
    <span class="nd">@abstractmethod</span>
    <span class="k">def</span> <span class="nf">my_abstract_method</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="o">...</span><span class="p">):</span>
        <span class="o">...</span>
    <span class="nd">@classmethod</span>
    <span class="nd">@abstractmethod</span>
    <span class="k">def</span> <span class="nf">my_abstract_classmethod</span><span class="p">(</span><span class="bp">cls</span><span class="p">,</span> <span class="o">...</span><span class="p">):</span>
        <span class="o">...</span>
    <span class="nd">@staticmethod</span>
    <span class="nd">@abstractmethod</span>
    <span class="k">def</span> <span class="nf">my_abstract_staticmethod</span><span class="p">(</span><span class="o">...</span><span class="p">):</span>
        <span class="o">...</span>

    <span class="nd">@property</span>
    <span class="nd">@abstractmethod</span>
    <span class="k">def</span> <span class="nf">my_abstract_property</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
        <span class="o">...</span>
    <span class="nd">@my_abstract_property</span><span class="o">.</span><span class="n">setter</span>
    <span class="nd">@abstractmethod</span>
    <span class="k">def</span> <span class="nf">my_abstract_property</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">val</span><span class="p">):</span>
        <span class="o">...</span>

    <span class="nd">@abstractmethod</span>
    <span class="k">def</span> <span class="nf">_get_x</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
        <span class="o">...</span>
    <span class="nd">@abstractmethod</span>
    <span class="k">def</span> <span class="nf">_set_x</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">val</span><span class="p">):</span>
        <span class="o">...</span>
    <span class="n">x</span> <span class="o">=</span> <span class="nb">property</span><span class="p">(</span><span class="n">_get_x</span><span class="p">,</span> <span class="n">_set_x</span><span class="p">)</span>
</pre></div>
</div>
<p>In order to correctly interoperate with the abstract base class machinery,
the descriptor must identify itself as abstract using
<code class="xref py py-attr docutils literal notranslate"><span class="pre">__isabstractmethod__</span></code>. In general, this attribute should be <code class="docutils literal notranslate"><span class="pre">True</span></code>
if any of the methods used to compose the descriptor are abstract. For
example, Python’s built-in <a class="reference internal" href="functions.html#property" title="property"><code class="xref py py-class docutils literal notranslate"><span class="pre">property</span></code></a> does the equivalent of:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="k">class</span> <span class="nc">Descriptor</span><span class="p">:</span>
    <span class="o">...</span>
    <span class="nd">@property</span>
    <span class="k">def</span> <span class="nf">__isabstractmethod__</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
        <span class="k">return</span> <span class="nb">any</span><span class="p">(</span><span class="nb">getattr</span><span class="p">(</span><span class="n">f</span><span class="p">,</span> <span class="s1">&#39;__isabstractmethod__&#39;</span><span class="p">,</span> <span class="kc">False</span><span class="p">)</span> <span class="k">for</span>
                   <span class="n">f</span> <span class="ow">in</span> <span class="p">(</span><span class="bp">self</span><span class="o">.</span><span class="n">_fget</span><span class="p">,</span> <span class="bp">self</span><span class="o">.</span><span class="n">_fset</span><span class="p">,</span> <span class="bp">self</span><span class="o">.</span><span class="n">_fdel</span><span class="p">))</span>
</pre></div>
</div>
<div class="admonition note">
<p class="admonition-title">Note</p>
<p>Unlike Java abstract methods, these abstract
methods may have an implementation. This implementation can be
called via the <a class="reference internal" href="functions.html#super" title="super"><code class="xref py py-func docutils literal notranslate"><span class="pre">super()</span></code></a> mechanism from the class that
overrides it.  This could be useful as an end-point for a
super-call in a framework that uses cooperative
multiple-inheritance.</p>
</div>
</dd></dl>

<p>The <a class="reference internal" href="#module-abc" title="abc: Abstract base classes according to :pep:`3119`."><code class="xref py py-mod docutils literal notranslate"><span class="pre">abc</span></code></a> module also supports the following legacy decorators:</p>
<dl class="function">
<dt id="abc.abstractclassmethod">
<code class="sig-prename descclassname">&#64;</code><code class="sig-prename descclassname">abc.</code><code class="sig-name descname">abstractclassmethod</code><a class="headerlink" href="#abc.abstractclassmethod" title="Permalink to this definition">¶</a></dt>
<dd><div class="versionadded">
<p><span class="versionmodified added">New in version 3.2.</span></p>
</div>
<div class="deprecated">
<p><span class="versionmodified deprecated">Deprecated since version 3.3: </span>It is now possible to use <a class="reference internal" href="functions.html#classmethod" title="classmethod"><code class="xref py py-class docutils literal notranslate"><span class="pre">classmethod</span></code></a> with
<a class="reference internal" href="#abc.abstractmethod" title="abc.abstractmethod"><code class="xref py py-func docutils literal notranslate"><span class="pre">abstractmethod()</span></code></a>, making this decorator redundant.</p>
</div>
<p>A subclass of the built-in <a class="reference internal" href="functions.html#classmethod" title="classmethod"><code class="xref py py-func docutils literal notranslate"><span class="pre">classmethod()</span></code></a>, indicating an abstract
classmethod. Otherwise it is similar to <a class="reference internal" href="#abc.abstractmethod" title="abc.abstractmethod"><code class="xref py py-func docutils literal notranslate"><span class="pre">abstractmethod()</span></code></a>.</p>
<p>This special case is deprecated, as the <a class="reference internal" href="functions.html#classmethod" title="classmethod"><code class="xref py py-func docutils literal notranslate"><span class="pre">classmethod()</span></code></a> decorator
is now correctly identified as abstract when applied to an abstract
method:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="k">class</span> <span class="nc">C</span><span class="p">(</span><span class="n">ABC</span><span class="p">):</span>
    <span class="nd">@classmethod</span>
    <span class="nd">@abstractmethod</span>
    <span class="k">def</span> <span class="nf">my_abstract_classmethod</span><span class="p">(</span><span class="bp">cls</span><span class="p">,</span> <span class="o">...</span><span class="p">):</span>
        <span class="o">...</span>
</pre></div>
</div>
</dd></dl>

<dl class="function">
<dt id="abc.abstractstaticmethod">
<code class="sig-prename descclassname">&#64;</code><code class="sig-prename descclassname">abc.</code><code class="sig-name descname">abstractstaticmethod</code><a class="headerlink" href="#abc.abstractstaticmethod" title="Permalink to this definition">¶</a></dt>
<dd><div class="versionadded">
<p><span class="versionmodified added">New in version 3.2.</span></p>
</div>
<div class="deprecated">
<p><span class="versionmodified deprecated">Deprecated since version 3.3: </span>It is now possible to use <a class="reference internal" href="functions.html#staticmethod" title="staticmethod"><code class="xref py py-class docutils literal notranslate"><span class="pre">staticmethod</span></code></a> with
<a class="reference internal" href="#abc.abstractmethod" title="abc.abstractmethod"><code class="xref py py-func docutils literal notranslate"><span class="pre">abstractmethod()</span></code></a>, making this decorator redundant.</p>
</div>
<p>A subclass of the built-in <a class="reference internal" href="functions.html#staticmethod" title="staticmethod"><code class="xref py py-func docutils literal notranslate"><span class="pre">staticmethod()</span></code></a>, indicating an abstract
staticmethod. Otherwise it is similar to <a class="reference internal" href="#abc.abstractmethod" title="abc.abstractmethod"><code class="xref py py-func docutils literal notranslate"><span class="pre">abstractmethod()</span></code></a>.</p>
<p>This special case is deprecated, as the <a class="reference internal" href="functions.html#staticmethod" title="staticmethod"><code class="xref py py-func docutils literal notranslate"><span class="pre">staticmethod()</span></code></a> decorator
is now correctly identified as abstract when applied to an abstract
method:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="k">class</span> <span class="nc">C</span><span class="p">(</span><span class="n">ABC</span><span class="p">):</span>
    <span class="nd">@staticmethod</span>
    <span class="nd">@abstractmethod</span>
    <span class="k">def</span> <span class="nf">my_abstract_staticmethod</span><span class="p">(</span><span class="o">...</span><span class="p">):</span>
        <span class="o">...</span>
</pre></div>
</div>
</dd></dl>

<dl class="function">
<dt id="abc.abstractproperty">
<code class="sig-prename descclassname">&#64;</code><code class="sig-prename descclassname">abc.</code><code class="sig-name descname">abstractproperty</code><a class="headerlink" href="#abc.abstractproperty" title="Permalink to this definition">¶</a></dt>
<dd><div class="deprecated">
<p><span class="versionmodified deprecated">Deprecated since version 3.3: </span>It is now possible to use <a class="reference internal" href="functions.html#property" title="property"><code class="xref py py-class docutils literal notranslate"><span class="pre">property</span></code></a>, <code class="xref py py-meth docutils literal notranslate"><span class="pre">property.getter()</span></code>,
<code class="xref py py-meth docutils literal notranslate"><span class="pre">property.setter()</span></code> and <code class="xref py py-meth docutils literal notranslate"><span class="pre">property.deleter()</span></code> with
<a class="reference internal" href="#abc.abstractmethod" title="abc.abstractmethod"><code class="xref py py-func docutils literal notranslate"><span class="pre">abstractmethod()</span></code></a>, making this decorator redundant.</p>
</div>
<p>A subclass of the built-in <a class="reference internal" href="functions.html#property" title="property"><code class="xref py py-func docutils literal notranslate"><span class="pre">property()</span></code></a>, indicating an abstract
property.</p>
<p>This special case is deprecated, as the <a class="reference internal" href="functions.html#property" title="property"><code class="xref py py-func docutils literal notranslate"><span class="pre">property()</span></code></a> decorator
is now correctly identified as abstract when applied to an abstract
method:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="k">class</span> <span class="nc">C</span><span class="p">(</span><span class="n">ABC</span><span class="p">):</span>
    <span class="nd">@property</span>
    <span class="nd">@abstractmethod</span>
    <span class="k">def</span> <span class="nf">my_abstract_property</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
        <span class="o">...</span>
</pre></div>
</div>
<p>The above example defines a read-only property; you can also define a
read-write abstract property by appropriately marking one or more of the
underlying methods as abstract:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="k">class</span> <span class="nc">C</span><span class="p">(</span><span class="n">ABC</span><span class="p">):</span>
    <span class="nd">@property</span>
    <span class="k">def</span> <span class="nf">x</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
        <span class="o">...</span>

    <span class="nd">@x</span><span class="o">.</span><span class="n">setter</span>
    <span class="nd">@abstractmethod</span>
    <span class="k">def</span> <span class="nf">x</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">val</span><span class="p">):</span>
        <span class="o">...</span>
</pre></div>
</div>
<p>If only some components are abstract, only those components need to be
updated to create a concrete property in a subclass:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="k">class</span> <span class="nc">D</span><span class="p">(</span><span class="n">C</span><span class="p">):</span>
    <span class="nd">@C</span><span class="o">.</span><span class="n">x</span><span class="o">.</span><span class="n">setter</span>
    <span class="k">def</span> <span class="nf">x</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">val</span><span class="p">):</span>
        <span class="o">...</span>
</pre></div>
</div>
</dd></dl>

<p>The <a class="reference internal" href="#module-abc" title="abc: Abstract base classes according to :pep:`3119`."><code class="xref py py-mod docutils literal notranslate"><span class="pre">abc</span></code></a> module also provides the following functions:</p>
<dl class="function">
<dt id="abc.get_cache_token">
<code class="sig-prename descclassname">abc.</code><code class="sig-name descname">get_cache_token</code><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#abc.get_cache_token" title="Permalink to this definition">¶</a></dt>
<dd><p>Returns the current abstract base class cache token.</p>
<p>The token is an opaque object (that supports equality testing) identifying
the current version of the abstract base class cache for virtual subclasses.
The token changes with every call to <a class="reference internal" href="#abc.ABCMeta.register" title="abc.ABCMeta.register"><code class="xref py py-meth docutils literal notranslate"><span class="pre">ABCMeta.register()</span></code></a> on any ABC.</p>
<div class="versionadded">
<p><span class="versionmodified added">New in version 3.4.</span></p>
</div>
</dd></dl>

<p class="rubric">Footnotes</p>
<dl class="footnote brackets">
<dt class="label" id="id2"><span class="brackets"><a class="fn-backref" href="#id1">1</a></span></dt>
<dd><p>C++ programmers should note that Python’s virtual base class
concept is not the same as C++’s.</p>
</dd>
</dl>
</div>


          </div>
        </div>
      </div>
      <div class="sphinxsidebar" role="navigation" aria-label="main navigation">
        <div class="sphinxsidebarwrapper">
  <h4>Previous topic</h4>
  <p class="topless"><a href="contextlib.html"
                        title="previous chapter"><code class="xref py py-mod docutils literal notranslate"><span class="pre">contextlib</span></code> — Utilities for <code class="xref std std-keyword docutils literal notranslate"><span class="pre">with</span></code>-statement contexts</a></p>
  <h4>Next topic</h4>
  <p class="topless"><a href="atexit.html"
                        title="next chapter"><code class="xref py py-mod docutils literal notranslate"><span class="pre">atexit</span></code> — Exit handlers</a></p>
  <div role="note" aria-label="source link">
    <h3>This Page</h3>
    <ul class="this-page-menu">
      <li><a href="../bugs.html">Report a Bug</a></li>
      <li>
        <a href="https://github.com/python/cpython/blob/3.8/Doc/library/abc.rst"
            rel="nofollow">Show Source
        </a>
      </li>
    </ul>
  </div>
        </div>
      </div>
      <div class="clearer"></div>
    </div>  
    <div class="related" role="navigation" aria-label="related navigation">
      <h3>Navigation</h3>
      <ul>
        <li class="right" style="margin-right: 10px">
          <a href="../genindex.html" title="General Index"
             >index</a></li>
        <li class="right" >
          <a href="../py-modindex.html" title="Python Module Index"
             >modules</a> |</li>
        <li class="right" >
          <a href="atexit.html" title="atexit — Exit handlers"
             >next</a> |</li>
        <li class="right" >
          <a href="contextlib.html" title="contextlib — Utilities for with-statement contexts"
             >previous</a> |</li>

          <li><img src="../_static/py.svg" alt="python logo" style="vertical-align: middle; margin-top: -1px"/></li>
          <li><a href="https://www.python.org/">Python</a> &#187;</li>
          <li class="switchers">
            <div class="language_switcher_placeholder"></div>
            <div class="version_switcher_placeholder"></div>
          </li>
          <li>
              
          </li>
    <li id="cpython-language-and-version">
      <a href="../index.html">3.8.13 Documentation</a> &#187;
    </li>

          <li class="nav-item nav-item-1"><a href="index.html" >The Python Standard Library</a> &#187;</li>
          <li class="nav-item nav-item-2"><a href="python.html" >Python Runtime Services</a> &#187;</li>
                <li class="right">
                    

    <div class="inline-search" role="search">
        <form class="inline-search" action="../search.html" method="get">
          <input placeholder="Quick search" aria-label="Quick search" type="text" name="q" />
          <input type="submit" value="Go" />
          <input type="hidden" name="check_keywords" value="yes" />
          <input type="hidden" name="area" value="default" />
        </form>
    </div>
                     |
                </li>
            
      </ul>
    </div>  
    <div class="footer">
    &copy; <a href="../copyright.html">Copyright</a> 2001-2022, Python Software Foundation.
    <br />
    This page is licensed under the Python Software Foundation License Version 2.
    <br />
    Examples, recipes, and other code in the documentation are additionally licensed under the Zero Clause BSD License.
    <br />
    
    <br />

    The Python Software Foundation is a non-profit corporation.
<a href="https://www.python.org/psf/donations/">Please donate.</a>
<br />
    <br />

    Last updated on Mar 17, 2022.
    <a href="https://docs.python.org/3/bugs.html">Found a bug</a>?
    <br />

    Created using <a href="https://www.sphinx-doc.org/">Sphinx</a> 2.4.4.
    </div>

  </body>
</html>